Generally, the so-called pyroelectric infrared sensors have been known and in use as infrared sensors for detection of infrared radiations from a heating element. Such pyroelectric infrared sensors employ an infrared detection element of a pyroelectric material such as lead titanate (PbTiO.sub.3) or the like which is capable of producing pyroelectric effects. Namely, a pyroelectric element has properties such that, when subjected to a temperature change, for example, when heated by infrared rays incident on its surface, it loses electrical stability and produces charge due to spontaneous polarizations which turn a neutral state of charge into an electrically unbalanced state. Since pyroelectric material has an extremely high impedance, e.g., as high as several hundreds G.OMEGA., the charge produced in an infrared detection element can be picked up as a voltaic output signal by the use of an impedance transformer.
Illustrated by way of example in FIG. 14 are an infrared detection element and an impedance transformer circuit in a prior art infrared sensor.
In this figure, indicated at 101 is a pyroelectric infrared sensor which is constituted by an infrared detection element 102 and an impedance transformer circuit 103. The just-mentioned impedance transformer circuit 103 consists of a field-effect transistor (FET) 104 having its gate terminal connected to the output of the infrared detection element 102, a gate resistor Rg connected between the gate terminal of FET 104 and ground, and a drain resistor Rd connected between the drain terminal of FET 104 and ground. FET 104 is supplied with a source voltage Vcc at its source terminal, delivering at its drain terminal an output signal Vs from the pyroelectric infrared sensor 101.
In this instance, the output signal Vs of the impedance transformer circuit 103 consists of two components (AC and DC components), more specifically, a weak AC signal from the infrared detection element 102 and a DC signal attributable to the nature of FET 104.
Illustrated in FIG. 15 is a prior art signal processor circuitry which, has been conventionally resorted to for amplification of output signal Vs of a pyroelectric infrared sensor.
As seen in this figure, the signal processor is arranged as an inverting amplifier circuit 111 including an OP AMP (operational amplifier) 112, a coupling capacitor Ca and an input resistor ra connected in series between an inverting input terminal of the OP AMP 112 and an input terminal T1 of the inverting amplifier circuit 111, and a negative feedback resistor rb connected between output terminal and inverting input terminal of OP AMP 112. The noninverting input terminal of OP AMP 112 is grounded.
The output signal Vs of the infrared sensor 101 is fed to the input terminal T1 of the inverting amplifier circuit 111 as an input signal Vin, which input signal Vin being fed to the inverting input terminal of OP AMP 112 through a series circuit of the coupling capacitor Ca and input resistor ra for removing DC components therefrom.
OP AMP 112 has its output terminal T2 connected to its inverting input terminal through the negative-feedback resistor rb. The output voltage Vout can be defined as in Expression 1 below where f is the frequency of the input signal Vin. ##EQU1##
As clear from Expression 1 above, the smaller the frequency (f) of the input signal Vin, the smaller become the amplification factor and the output voltage Vout. Therefore, in order to use the inverting amplifier circuit 111 at a predetermined amplification factor, it is necessary for the input frequency (f) to be in a limited bandwidth.
In this regard, irrespective of the frequency (f) of the input signal Vin, it is conceivable to obtain a predetermined amplification factor through the inverting amplifier circuit 111 by increasing the resistance of the input resistor ra to a much greater value as compared with the impedance (1/2.pi.Ca) (.OMEGA.) of the capacitor Ca or by increasing the electrostatic capacitance of the capacitor Ca. For instance, in a case where the electrostatic capacitance of the capacitor Ca is set at 22 (.mu.F), the amplification rate at 60 (dB) and the low-frequency cut-off level (f) at 0.1 Hz, respectively, it will become necessary to employ an input resistor ra of approximately 70 (k.OMEGA.) and a negative-feedback resistor rb of approximately 70 (M.OMEGA.).
Moreover, for the purpose of broadening the viewfield of detection, there may arise a necessity for using a pyroelectric infrared sensor which consists of a plural number of similar pyroelectric infrared sensor units 101a, 101b, . . . 101n. In such a case, it has been the usual practice to feed input signals Vin from a plural number of pyroelectric infrared sensor units 101a to 101n sequentially to a common inverting amplifier circuit 111 by way of switches 113a to 113n for amplification as shown in FIG. 16, because this is desirable from the standpoint of simplification of circuit arrangements and from reducing the number of component parts and the production cost.
However, difficulties are usually encountered in case the negative-feedback resistor rb of the inverting amplifier circuit 111 is required to be of a resistance value as high as 70 M.OMEGA., because resistors of this class are very costly despite irregularities in resistance value which will be reflected by irregularities in characteristics among individual amplifier circuits of ultimate products.
Besides, it becomes necessary to use an electrolytic capacitor for the coupling capacitor Ca which needs to be of a large electrostatic capacitance. However, the use of an electrolytic capacitor involves a problem of large leakage current through the input resistor ra and the negative-feedback resistor rb, inviting a problem of offset voltage which occurs to the output voltage Vout after amplification due to a voltage drop.
Further, in case an electrolytic capacitor of normal standards is used in combination with a negative feedback resistor rb of 70 (M.OMEGA.) in resistance value, the output voltage Vout tends to get saturated under the influence of the leakage current, rendering the operation of the inverting amplifier circuit completely infeasible. For these reasons, difficulties have been experienced in amplifying an input signal Vin of low frequency (f) at a high amplification factor.
Furthermore, in a case where a coupling capacitor Ca is connected to the inverting input terminal of OP AMP 112, due to a DC potential difference across the electrodes of the capacitor, the inverting amplifier circuit 111 cannot be put in a normally operative state immediately after turning on a power switch, that is, it cannot be put in a normally operative state until charge is stored in the capacitor Ca up to a level corresponding to the potential difference.
For example, in the case of the inverting amplifier circuit shown in FIG. 15, considering the large electrostatic capacitance and the time constant, the capacitor Ca takes at least 20 seconds to reach a charged state even if arrangements are made to shorten its charging time. Therefore, difficulties have also been experienced in amplifying output signals Vs of a plural number of pyroelectric infrared sensors 101a to 101n by sequentially connecting the respective infrared sensors 101a to 101n operatively to a single common inverting amplifier circuit 111 by a high speed switching operation.
It is an object of the present invention to provide a signal processor for a pyroelectric infrared sensor or sensors, which signal processor being capable of amplifying a low-frequency output signal of a pyroelectric infrared sensor at a constant amplification factor, without resorting to a resistor of a high resistance value or a coupling capacitor.